1. Field of the Invention
The present invention relates to mitigating cellular telephone interference. More specifically, the present invention relates to utilizing blanking to mitigate interference in electrical devices due to RF pulse transmissions from TDMA-based telephones.
2. Background of the Invention
In 2006, there were an estimated two-billion cellular telephones worldwide. Today there are countless cellular telephones in use. Cellular telephones communicate with a network of cellular towers to transmit calls between users. This network of cellular towers includes many towers in many places, but the number of cellular telephones grossly outweighs them. In order for these cellular towers to communicate with each cellular telephone individually, some form of multiplexing needs to be employed.
Multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. This process combines multiple signals and sends them as a single, complex signal. For analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into sub-channels of different frequency widths, each carrying a signal at the same time in parallel. For digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in individual time slots. Once the complex time-division signal reaches its destination, a de-multiplexer is used to separate the combined signals.
Time-division multiplexing is the basis for the standard in cellular communications known as Time Division Multiple Access (TDMA), which is used by Digital-American Mobile Phone Service (D-AMPS), Global System for Mobile communications (GSM), and Personal Digital Cellular (PDC). Of these, the GSM standard is the most commonly used having over 120 million users, offered in over 120 countries, and is the standard in Europe.
A cellular telephone using the GSM standard employs a radio frequency (RF) transmitter to communicate with the nearest cellular tower. During this communication the RF transmitter turns on and off at a fast rate of about 217 Hz. This high frequency switching on and off is known as the RF transmitter's pulse repetition rate, which has a pulse width, or burst, of about 577 microseconds (μs). This RF transmission burst can be unintentionally received by nearby electronic devices that do not have proper RF shielding, or are connected to other devices that do not have proper RF shielding. When this nearby electronic device is an audible device, such as a television, hearing aid, or anything with a speaker or headphone, the burst can be heard by a human. Typically, the human ear can pick up frequencies that range from roughly 20 to 20,000 Hz. Not everyone has exactly the same range, but a frequency of 217 Hz can be easily heard by most people.
Many have had the experience of hearing a buzzing sound emitting from a nearby speaker seconds before they receive a phone call. There are a number of factors involved that dictate this occurrence. The farther a cellular telephone is from the nearest tower, the more powerful signal it must produce to communicate with it. A more powerful signal has a greater likelihood of interfering with nearby electronic devices. Also, the closer the electronic device is to the cellular telephone the more likely the electronic device will be unintentionally affected by the phone's burst transmissions. If the electronic device is hard-wired to the cellular telephone, such as a hands-free ear piece, it can pick up the burst transmission even more easily. The power from the RF transmitter's direct current (DC) supply circuitry will fluctuate due to the relatively large power consumption from the RF transmitter turning on and off. The power to the transmitter also fluctuates at 217 Hz, and this can be picked up by the ear piece, making it difficult to hear. In some cases the microphone of the cellular telephone may also be affected, making it difficult for the other person to hear as well.
Medical equipment, while not necessarily having speakers attached, can be sensitive to the frequencies it detects, and is dependent upon them for results. Interference with these sensitive frequencies can skew results, which can lead to misdiagnoses and resultant harm or death of the patient. As a result, many hospitals do not allow, or discourage, the use of cellular telephones except in designated areas.
Presently, there exist a few ways to avoid this interference. One method is to simply move the cellular telephone away from the affected electronic device. The distance between the cellular phone and the electronic device determines the amount of interference, which translates into the audible volume of the interference through an affected speaker. The cellular telephone may be moved to a distance where the interference is no longer present, or at least to where the interference can no longer be heard. This method may work if the interfering device is known, and can be avoided, but this is not always the case. Many times the user may be standing in a position where the signal is the greatest between the cellular telephone and the tower and any change in location may be suboptimal or even nonfunctional. Also, this method will not work when the affected device is attached to the user, such as with a hearing aid, or any device hard-wired into the cellular telephone. In the case of medical devices, the problem is not merely annoyance, but a change in the results. Since this cannot be detected by the user, a change in location does not appear to be necessary and the user has no way of knowing how far away to move. Furthermore, medical devices can be affected by less interference than what would be audible. Certain medical devices must have no interference at all in order to operate properly. This requires the user to move a greater distance from a medical device than a speaker.
Another method of avoiding RF burst transmission interference is to employ the proper amount of shielding in electronic devices that can be affected by this interference. Shielding of electronic devices and cables includes creating a mesh of interwoven conductive fiber such as copper, iron, or even gold. The fiber mesh acts as a series of loops which dissipates the signal before it reaches the underlying electronic device or cable. Though simple in architecture, this type of shielding is expensive. Even if an electronic device is properly shielded, it may receive interference anyway through any hard-wired device or cable that is not properly shielded. Just one unshielded cable can cause every electronic device to which it's connected, and every device to which those devices are connected, to receive the interference that the unshielded cable receives. This solution will not cure interference with a device that is hard-wired to the telephone either.
The RF transmitter in a cellular telephone is only turned on when making or receiving calls. Either the tower sends a signal to cellular telephone telling it to activate its RF transmitter, or the user elects to place a call. In both cases, the cellular telephone must follow a specific protocol when setting up the call, and this results in a situation where the phone is aware of precisely when its RF transmitter will be activated. What is needed in the art is a system and method of alerting nearby electronic devices of impending interference based on the phone's knowledge of when it will transmit. This alert should be received by nearby electronic devices in time to compensate for the interference.